Various AC frequency conversion techniques are known in the art. One technique, called the inverterconverter approach, or the step converter method, rectifies the AC voltage through a diode rectifier bridge and smooths the rectified AC to level DC by means of a capacitor or the like. This level DC is then used to synthesize a series of incremental steps to approximate a given sine wave voltage of desired frequency, for example "Power Transistor Applications for Switching Regulators and Motor Control", Marvin W. Smith, General Electric Co., Semiconductor Products Dept., Auburn, N.Y., Oct., 1979, pages 22-23.
The present AC frequency conversion technique eliminates the capacitive filtering to DC. The present invention further eliminates the rectifier bridge by enabling a switched output to also perform the rectifying function by means of irregular switching times. In preferred form, a chopped sinusoid output waveform is provided by switching between a pair of power lines at a given frequency and also switching between the power lines in response to each zero crossing of the AC signal except when the zero crossing and the given frequency clock signal coincide in time.
The invention in particularly useful for up conversion in certain motor control applications, specifically where an increase in frequency is desired for only short periods of time compared with normal low frequency run-time. An example is refrigeration control where the compressor must be designed for the worst case situation even though such worst case occurs perhaps only 1% of the time, for example when a freezer must cool down a whole new supply of food. During the other 99% of the time, the compressor must only maintain an already cool condition, and thus may only need perhaps half its capacity. One solution to this over-capacity is to use a smaller compressor and run it at normal speed for normal duty, and run it at a higher speed during the small percentage of time needed for higher capacity cooling, i.e. during the 1% cool-down time. This faster speed operation is not detrimental to the compressor for short periods of time.
In the present invention, the AC frequency may be increased in a simple manner for running the compressor at a faster speed. A trade-off in the present frequency conversion technique is that the resultant chopped sinusoid switched output waveform of increased frequency is less efficient than the input AC frequency. This less efficient use of electrical power is far outweighed by the reduction in compressor capacity enabled thereby. Furthermore, during the 99% normal run-time, a smaller compressor is driven by a smaller motor at its most efficient load rating.